The beat-to-beat function of the left ventricle as it changes function from one steady-state level to another in response to disturbances such as postural changes, exercise, and ventilation of the lungs may not be completely described by more traditional studies of steady-state function. Potential differences between transient and steady-state descriptions of left ventricular function are especially important for realizing the potential utility of the end-systolic pressure-volume relationship in the clinical assessment of left ventricular pump function. The focus of this proposal is a potentially important difference between transient and steady-state left ventricular function that has become apparent in recent years: that is, the muscle of the left ventricle "remembers" its history of contractile function on previous beats. The general goal of this proposal is to confirm the importance of previous beat history dependence in the hemodynamic function of the whole left ventricle of the dog, assess its importance relative to traditional determinants of cardiac function, and determine its sensitivity to changes in heart rate and inotropic state. Projects I and II use a deterministic experimental approach to test a related group of hypotheses about the importance of previous beat ejection (i.e., shortening) on left ventricular function at different heart rates and levels of inotropic state. Projects III and IV use a more statistical modeling approach for studying a similar group of related hypotheses about the importance of previous beat hemodynamic memory in response to sudden perturbations in left ventricular preload and afterload at different heart rates and different levels of inotropic state.